Re: bpf_fib_lookup VLAN

On 4/16/21 9:12 AM, Konrad Zemek wrote:
> (Sorry for the double mail David, I missed the "reply all" button).
> 
> On Friday, April 16, 2021 5:35 PM, David Ahern <[email protected]> wrote:
>> On 4/16/21 5:03 AM, Konrad Zemek wrote:
>>
>>> So both vlan fields in the output struct bpf_fib_lookup are always zero. I
>>> haven't seen this commented on anywhere, including the discussion around
>>> introducing bpf_fib_lookup, so I assume it's an accidental oversight.
>>
>> The uapi was setup to cover the use case, but VLANs are not supported at
>> the moment.
> 
> I'm surprised it's not marked as such in the bpf.h, the comments (or rather
> lack of) made me convinced that it works just as well as the MAC address
> fields.
> 
>> On 4/16/21 5:03 AM, Konrad Zemek wrote:
>>> Do you have any proposals for a workaround? Right now I'm thinking of
>>> creating a BPF map that would map ifindex->vlan, populated in the userspace
>>>
>>> -   but that assumes the output (struct bpf_fib_lookup*)->ifindex will be an
>>>     index of the vlan device and not the physical device the vlan is attached
>>>     on, which I'm not sure is the case yet.
>>>
>>
>> vlan netdevices do not support XDP redirect.
>>
>> It's not a trivial problem to handle VLANs or stacked devices in
>> general. I have working code here:
>>
>> https://github.com/dsahern/linux/commits/bpf/mpls-vlan-fwd
>>
>> but it is not ready for submitting upstream yet. The use case and
>> related ones need more work.
> 
> They don't, but it's not that important for my use case. I have just
> one interface and all the VLANs are on that, so if I learn that a
> VLAN is needed it's just another thing I push in front of the tunnel
> frames that I already push. If I had multiple interfaces, I'd just
> need one more piece of info which is "what's the physical interface
> number this VLAN is attached on".

Your use case is fairly trivial to support, but generically the vlan can
be on a bridge, bond, or port. To properly support VLAN redirects, the
fib lookup needs to resolve the stack to the egress port.

> 
> XDP programs are already pretty specific to the infrastructure one's
> running, and already very manual with packet manipulation (which is
> actually a boon to a lot of things I'm doing), so I don't mind this
> not being a generic solution.
> 

As I recall (it has been a few years), the fib lookup device index is
the vlan device. You could have a map that converts that return to the
real port and vlan. It should work, just more maintenance.